1. Field of the Invention
The present invention relates to ink jet printers. In particular, the invention relates to methods and apparatus for enhancing the reliability of ink jet printer heads.
2. Prior Art
The use of ink jet printers for printing information on recording media is well known in the prior art. Conventional ink jet printers incorporate a plurality of electrical components and fluidic components. The components coact to perform the printing function.
The fluidic components include a drop generator having a chamber for affecting drop inducing vibration on a printing fluid or ink and a nozzle plate with one or more ink nozzles interconnected to the chamber. A gutter assembly is positioned downstream from the nozzle plate in the flight path of ink droplets. The gutter assembly catches ink droplets which are not needed for printing on the recording medium.
In order to create the ink droplets, an electrical transducer within the drop generator vibrates at a frequency which forces the thread-like streams of ink which are initially ejected from the nozzles to be broken up into a series of ink droplets at a point within the vicinity of the nozzle plate. A charge electrode is positioned along the flight path of the ink droplets. The function of the charge electrode is to selectively induce a charge on the ink droplets as said droplets separate from the stream. A pair of deflection plates is positioned downstream from the charge electrodes. The function of the deflection plates is to deflect a charged ink droplet either into the gutter or onto the recording media.
One of the most pressing problems associated with ink jet printers of the above described type is that of head reliability. Most of the head failures occur at the instant when the heads are turned on (that is, stream start-up) or turned off (that is, stream shut-down). It is believed that temporary stream instability is the prime cause of these reliability problems.
The causes for the stream instability are the start-up/shut-down dynamics and contamination associated with the streams. The term start-up/shut-down dynamics is used to describe any form of sputtering, oozing, low velocity or misdirected ink stream. Among other things, these aberrations of the ink stream stem from the presence of air bubbles in the head and slow ink pressure transition within the head at start-up or shut-down. Contamination results in partial or complete blocking of the nozzle hole which results in stream misdirection.
As was stated previously, the ink streams and/or ink droplets are projected through several electrode structures for deflection. The maximum clearance between the electrode structures and the ink stream and/or ink droplets is typically 0.015 inch. With this tight clearance, any sputtering or oozing etc. of the stream results in wetting the electrodes and ultimately electrical shorting.
One method described in the prior art to alleviate the above described problem is the so-called "HARD START" method. This is accomplished with a high performance valve positioned in the nozzle head. The valve causes the pressure transition in the head to occur in sub-millisecond times. This approach largely avoids stream dynamics type failures. However, failures associated with stream blockage (contamination) are not addressed. Also a highly tuned valve is needed which tends to increase the overall cost of the head and additionally this approach places constraints on other drop generator components which tend to limit design freedom. Finally, significant measures must be taken to ensure that no air is allowed to enter the head cavity.
U.S. Pat. No. 3,839,721 discloses a method and apparatus used to prevent ink from drying at the nozzle during printer shut-down and to keep the charging electrode and deflection plates free from ink spraying at pressure shutoff. In addition to the conventional gutter structure associated with an ink jet printer, a second gutter-like structure having a vapor chamber and with an opening having a partially closed lip portion is positioned between the charge electrodes and the deflection electrodes. At shutdown time the charge electrodes are moved up out of the path of the jet streams and the second gutter-like structure is moved into the jet streams along a path transverse to the flight path of the droplets of the jet stream. In this position, ink issuing from the nozzle is caught by the gutter.
Although this prior art is a satisfactory approach for its intended purpose, one of its shortcomings is that splashing of ink is not completely eliminated since the closed lip portion of the gutter-like structure crosses the flight path of active ink streams.
U.S. Pat. No. 4,031,561 discloses another technique used in the prior art to solve the start-up and/or shut-down problem. According to the teachings of the patent, at start-up time, the charge plate is positioned to within 0.005 millimeters of the orifice plate which supports the ink jet nozzles. A purge liquid is used to flush the ink jet nozzle until the ink streams are properly established. Thereafter the purge fluid is replaced with ink. The lower surface of the charge plate is plated with a nonwetting coating. The purge liquids which accumulate on the lower surface are dried by blowing air on said surface.
Other prior art techniques require the use of a wiping device for drying ink from the nozzle and/or electrodes. Still other prior art methods require the use of a cap or nozzle that move over the nozzle orifice at shut-down and/or start-up time. Detailed description of these techniques and methods are given in U.S. Pat. Nos. 3,945,020, 4,045,802 and IBM Technical Disclosure Bulletin Vol. 20, No. 2, July 1977, pgs. 786-788, and IBM Technical Disclosure Bulletin Vol. 18, No. 6, May 1976, pgs. 4138-4139.
Yet another technique used in the prior art to eliminate wetting of the electrode is disclosed in IBM Technical Disclosure Bulletin Vol. 18, No. 6, November 1975, pgs. 1813-1814. In the publication, the nozzles are aimed away from the charge and deflection electrodes at start-up and/or shut-down time.